For the recovery of selected constituents from a gas mixture it is known to utilize the so-called pressure-swing technique in which the initial gas mixture is alternately or cyclically fed to two or more adsorbers designed to retain the unwanted component or components; see, for example, U.S. Pat. No. 4,299,595. For the separation of nitrogen from a gas mixture such as air, in particular, it is known to use reactors containing adsorber beds in the form of carbonaceous molecular sieves--such as comminuted coke--which have a high affinity for oxygen and also tend to retain water vapor and carbon dioxide. With two alternately activated reactors of this character, the gas mixture (referred to hereinafter as air) is passed under high pressure through each adsorber-reactor in a respective production phase occupying a major part of a half-cycle of an operating cycle of predetermined duration, e.g. two minutes. While the active reactor discharges high-purity nitrogen as a product gas through an outlet thereof during the production phase of one half-cycle, its companion reactor is in a regeneration phase during which its internal pressure is lowered to or below atmospheric level by an at least partial removal of its gas content through venting or exhaustion. During the remaining minor part of that half-cycle, generally lasting for only a matter of seconds, the hitherto inactive companion reactor is preloaded with gas in a pressure-buildup phase immediately following its own regeneration phase and preceding the production phase of this reactor which starts at the beginning of the next half-cycle.
Naturally, the rate of the air flow admitted to either reactor and the capacity of its adsorber bed should be so correlated that, at the end of the chosen production phase, the active reactor should still be sufficiently unsaturated to keep the residual oxygen of the product gas below a maximum permissible level of, say, 1,000 parts per million (ppm). In order to increase the efficiency of a given adsorber bed, it has already been proposed to preload the previously inactive reactor before the beginning of its production phase with a gas whose initial nitrogen content is higher than that of air, namely some of the high-pressure gas still present in the interstices of the molecular sieve of the reactor whose production phase has just been terminated. This involves a temporary interconnection of the two paired reactors in a closed circuit, for an interval of several seconds representing an equalization period which forms part of the pressure-buildup phase of the just-regenerated reactor and during which the pressure differential between the two reactors drives the nitrogen-enriched gas into the reactor about to be activated.